Electrodes are used in energy storing devices, including but not limited to, batteries, capacitors, ultra-capacitors, non-aqueous-type secondary batteries and such.
Currently there are two primary means for producing electrodes: a “wet” method and a “dry” method. In the wet method, a polymeric binder in the form of a solvent solution or dispersion is blended with one or more active powdery electrode forming materials to form a slurry dispersion or paste. This dispersion or paste is then applied to one or both surfaces of an electroconductive substrate, and dried to form a coherent composite electrode layer. The electrode layer may then be calendered. This method is shown in U.S. Pat. Nos. 5,776,637 and 6,200,703, where a fluoropolymer binder is dissolved in NMP. An improvement on the solvent solution is the use of an aqueous fluoropolymer dispersion as the binder, as described in US 2010/0304270 and US 2012/0015246. Advantageously, in the aqueous dispersion process, the polymer binder stays as small discrete particles—and those particles bind the active electrode materials only at discrete points—resulting in an interconnectivity of the active electrode particles, rather than as the coating formed on the active particles from a polymer solution. Since most of the active electrode material is not covered by the binder, there is less insulation effect, less heat build-up, and better conductivity. Additionally, an aqueous binder provides environmental benefits over organic solvents, and less binder can be used since the dry active powdery electrode-forming materials are essentially spot-welded, rather than completely coated.
The second process for forming electrodes is by a dry process. In the dry process, a dry polymer powder is dry-blended with dry active powdery electrode-forming materials—often under high shear where the mixture is fibrillized—and the dry blend is either formed into a film which is adhered to an electroconductive substrate, or else the dry blend is applied directly to, and pressed onto the electroconductive substrate by means such as calendaring. The dry electrode process is described in U.S. Pat. Nos. 7,791,860, 7,791,861, 8,072,734, 8,591,601, and US 2011/0165318.
A problem with the wet method, is that a large amount of solvent must be evaporated—which has negative environmental effects, and also requires a lot of energy to remove.
A problem with the dry method is that dry particles of the binder polymer tend to agglomerate during the drying process or during storage, particularly if the binder particles are soft. The agglomerated polymer binder particles are difficult to distribute uniformly throughout the blend. This is especially an issue when low amounts of binder are desired to maximize performance, and also minimize cost.
There is a desire to obtain the small particle size and good distribution of the polymer binder, as found in an aqueous dispersion coating process, but with the advantages of reduced energy consumption of drying and little or no environmentally undesirable solvent waste, as found in the dry process.
Surprisingly, it has now been found that a hybrid process can provide advantages over both the wet and dry processes, leading to synergistic positive properties of the formed electrode. In the hybrid process, an aqueous dispersion of binder is added to dry active electrode-forming materials, that good distribution of small, discrete fluoropolymer binder particles occurs, and the water added with the dispersion is easily removed with heat applied during the electrode-forming process. Advantageously, the dispersion is more uniform, and the dispersed polymer particles do not agglomerate, providing the best of both processes.
Other advantages include: lower cost of manufacturing, energy saving, and better dispersion of the binder, which in turn could reduce binder usage.